An alarming proportion of American women enter pregnancy obese. Obese women are more likely to experience delayed onset of lactogenesis (late onset of copius milk production) and early breastfeeding cessation. While there has been much speculation, there is virtually no published research examining why obese women have poorer lactation outcomes. The applicants are in a unique position to address this issue, having access to an extensive collection of longitudinal data and biologic specimens from the Early Lactation Success study. In the primary analysis of the Early Lactation Success study, significant independent risk factors for delayed onset of lactogenesis (n=405 breastfeeding women with complete data;32% obese) were greater maternal body mass index, older maternal age, greater infant birth weight, and use of infant formula. Three of these risk factors (greater maternal body mass index, older maternal age, greater infant birth weight) are known correlates with maternal carbohydrate intolerance. Based on these findings, we hypothesize that metabolic disturbances associated with maternal carbohydrate intolerance contribute to delayed onset of lactogenesis. In further analysis, we observed that among the subset of women in the Early Lactation Success study with strong prenatal intentions to exclusively breastfeed (n=194), lean women were significantly more likely to be doing so at 2 months postpartum as compared to obese women. We hypothesize that in addition to greater risk of delayed onset of lactogenesis, obese women are more likely than non-obese women to experience several other early lactation difficulties, all contributing to shorter exclusive breastfeeding duration. We will test these hypotheses through 2 aims: 1) Examine the relationship between maternal carbohydrate intolerance and delayed onset of lactogenesis, and the extent to which carbohydrate intolerance explains the known association between maternal obesity and delayed onset of lactogenesis;and 2) Identify differences in the early breastfeeding experience between obese and non-obese women and the extent to which these differences explain the known link between maternal obesity and short exclusive breastfeeding duration. PUBLIC HEALTH RELEVANCE: Achieving the aims described in this application will inform the development of interventions designed to improve early breastfeeding outcomes among obese women. These efforts potentially have great public health impact given the high prevalence of obesity among women of reproductive age in the United States and the important advantages associated with longer episodes of lactation.